1. Field of the Invention
The present invention relates to a method for manufacturing a CMOS image sensor, and more particularly, to post-processing of a pad that includes a metal interconnection layer.
2. Description of the Related Art
The characteristics of the CMOS image sensor depend on the sensitivity of a photo diode receiving an external photo particle. This sensitivity depends largely on a distance and membrane characteristics between a microlens and a photo diode in alignment therewith.
However, a passivation nitride of a conventional pixel block does not reflect a light as much as a metal layer, but it blocks and reflects a relatively large part of the light that would otherwise pass to a passivation oxide. Accordingly, the sensitivity of the CMOS image sensor may deteriorate.
Meanwhile, in the case of the CMOS image sensor generally, a hydrogen anneal process for improving the low illumination characteristics (which differs from a general semiconductor manufacturing process) is performed not after laminating the passivation oxide, but after laminating the passivation nitride for further improving the low illumination characteristics. This raises various potential problems in the wafer edge region, related to a weak passivation oxide and an interlayer stress.
Hereinafter, referring to FIGS. 1a to 1h, a method for manufacturing a conventional CMOS image sensor (and problem[s] thereof) is described as below. In particular, FIGS. 1a to 1h represent processes from the step of forming microlenses to the step of opening a pad (after forming the pad).
Firstly, in FIG. 1a, after a pad 110 is formed, a passivation oxide and a passivation nitride are formed. On the other hand, in a wafer edge region, a wafer edge remaining membrane 100 of non-uniform state exists. The wafer edge membrane 100 is typically generated from a wafer edge treatment (e.g., through photolithographic processes) that differs from the same processes (at least in results) in the main chip region (e.g., the inner region of the wafer. Since the wafer edge membrane 100 is formed non-uniformly, the wafer edge region may have little or no adhesive strength.
As shown in FIG. 1b, in the case of the CMOS image sensor, a hydrogen anneal is performed, which comprises a thermal reflow using a gas mixture which typically includes hydrogen gas and nitrogen gas, in a proper or predetermined ratio.
In the thermal reflow, the interlayer stress of the passivation nitride 130 on the wafer edge area 100 may deteriorate, which can result in wafer edge peeling 140, which the passivation nitride 130 on the wafer edge can float in a circular shape and be raised since a fluorine ion in a fluorinated silicate glass (FSG) at the wafer edge membrane 100 may also “float.”
Moreover, in the thermal reflow, a peeling defect from the wafer edge can transition to a pixel region in the inner region of the wafer (e.g., that part of the wafer other than the outer 0.1-0.5 mm edge of the wafer). A circle defect 145 denotes a particle transferred to the inner region of the wafer.
As shown in FIG. 1c, the pad region 110 is opened through photolithography and etching processes after scrubbing in order to remove the circle defect 145. Cleaning and pad treatment processes, which include a pad ashing step, a solvent cleaning step, and a final cure step, are also performed.
Although the scrubbing process is performed to remove the circle defect 140, there is the circle defect 145 which is still not removed.
As shown in FIG. 1d, a pad protection membrane 150 is formed. The pad protection membrane 150 is a plasma enhanced tetraethyl orthosilicate (PETEOS) membrane or a thermosetting resin membrane, and may have a depth of 200 to 600 Å so that the pad 110 is opened easily.
FIG. 1e shows a color filter array 160 formed by one or more color filter photolithography processes.
FIG. 1f shows a planarization layer 170 formed by deposition of a planarization material, then patterning by a planarization layer photolithography process, and FIG. 1g shows a plurality of convex microlenses 180 formed by thermal reflow.
Finally, FIG. 1h shows an open pad 110, formed by etching the pad protection membrane 150 in the pad regions. Subsequently, a probing test is capable of being performed.